Cytokinesis, the cleavage of a mother cell into two daughters, is one of the most fundamental cellular processes yet it is poorly understood. The regulation and execution of cytokinesis are central to all aspects of animal development and to many pathologies such as cancer. Animal cell cytokinesis involves the progressive constriction of an equatorial ring whereas plant cell cytokinesis involves the assembly of a cell plate on a phragmoplast (a specialization of the spindle mid-body). Each of these mechanisms has been considered to be distinct. In this proposal it is hypothesized that the terminal phase of animal cell cytokinesis in fact uses a similar mechanism to that used in plant cytokinesis. In this view, cytokinesis consists of two phases: an initial furrowing phase that is independent of any mid-body microtubules and a terminal phase that involves the mid-body apparatus acting to finally pinch off and separate the two daughter cells in a manner analogous to the function of the phragmoplast of plants. It is proposed to explore this hypothesis by studying the terminal phase of cytokinesis in Caenorhabditis elegans using a combination of cell biological, structural and genetic strategies. Three main areas of study are proposed: 1) Studies of the role of septins in late cytokinesis. Septins are a class of structural protein initially identified as being required for cytokinesis in budding yeast (which do not have cleavage furrows) and subsequently shown to be required for animal cell cytokinesis. It is proposed to investigate the role of septins by antisense gene inactivation, ultrastructural localization of septins with respect to the microtubules of the spindle mid-body in normal embryonic cells together with those of mutants that fail in the late phase of cytokinesis, and isolation of suppressors or enhancers of septin mutants in order to identify interacting proteins. 2) Studies of the possible roles of calmodulin and myosin light chain kinase in the regulation of the late phase of cytokinesis by the use of specific photoactivatable inhibitors of these proteins. The inhibitors will be photoactivated in the spindle mid-body region. 3) Studies to determine whether secretion is required for the terminal phase of cytokinesis by looking for evidence of secretion in the vicinity of the mid-body at an ultrastructural level and by blocking secretion by drugs to see whether this perturbs the late phase of cytokinesis.